1. Field of the Invention
The present invention relates to a system to control the combining of two fluids and, more particularly, to such a system which automatically controls the volume proportion one fluid to another prior to the combining of the two fluids.
2. Setting of the Invention
In many situations it is necessary to precisely control the proportion of one fluid to another in a final mixture. One such situation is in the introduction of fluids into a wellbore for increased mobility of residual oil and/or for maintaining a desired pressure within a producing formation. For example, in certain reservoirs it has been found that fluids need to be injected into the producing formation to prevent condensate formation. In these situations an inert gas, such as nitrogen, in combination with a dry hydrocarbon gas is injected under pressure into the formation to prevent condensate formation. It is necessary to control the ratio or proportion of the inert gas to the hydrocarbon gas in the final mixture to assure that the inert gas does not damage the reservoir.
In the above situation, difficulties have arisen in controlling the proportion of inert gas to the hydrocarbon gas because these fluids are pressurized to greater than 6000 psi and thereby there is uncertainty as to the accuracy of metering the flow rates at these high pressures, since these fluids are then at greater than nine times their critical point. Other difficulties include inaccurate proportion control by manual operation of mixing valves in response to changes in flow conditions due to operator inattentiveness or fatigue. One solution to this problem is to provide alarms so that when the flow of inert gas and the hydrocarbon gas exceed or fall below a predetermined optimum level, then the alarm will be activated so the operator can take corrective action; however, this solution does not provide the accurateness required in some situations.
Various microprocessor based control systems have been developed for automatically controlling flow of fluids. One such system is described in U.S. Pat. No. 4,267,885 to Sanderford. In Sanderford, the temperature of a produced fluid at the wellhead is measured and is then provided to a process control unit for use in determining the injection parameter values needed to optimize well production. The process control unit, such as a microprocessor, is programmed to interpret the temperature data and to control a high pressure gas control valve. The patent of Sanderford is concerned only with monitoring temperature, and nowhere is it suggested or disclosed in Sanderford to have a system to control the combining of two fluids prior to injection into the wellbore to prevent a condensate formation. Further, the patent to Sanderford does not disclose or suggest the use of remote terminal units (RTU) in radio communication with a central or master terminal unit (MTU) to control the combining of two fluids prior to injection into a wellbore.
Another system to control the injection of fluids into a wellbore is described in U.S. Pat. No. 4,374,544 to Westerman, et al. In Westerman the fluid injection rates during secondary recovery injection operations are controlled by monitoring the bottomhole pressure adjacent the formation while adjusting the fluid injection rate for optimum recovery. Nowhere is it disclosed or suggested in Westerman to have a system to control the combining of two fluids prior to injection into a wellbore. Also, nowhere is it disclosed or suggested in Westerman to use remote terminal units in radio communication with a central or master terminal unit to control the combining of two fluids prior to injection into a wellbore.
Another system used to control the flow rates of fluids being injected into a wellbore is disclosed in SPE Paper 11111 by Charles P. Reese, published in September 1982. Within Reese's paper, a system is described which uses a microprocessor to control the production cycling of a well by continuous adjustment, based upon the changes in an individual well's producing pressures and flow rates. Nowhere is is disclosed or suggested within the Reese paper to control the combining of two fluids prior to injection into a wellbore.
In the above-described systems to control flow rates, a problem often encountered in the field is that the fluid compressor stations, valves, wellbores, etc., are remotely located from each other. These systems, if used in conjunction with remotely located master terminal units to oversee the overall operation of a series of these systems, requires the laying of cable or the use of phone lines to a control station. Laying cable is extremely expensive, as well as the cable oftentimes being cut or damaged by accidents. Further, phone lines are nonexistent in many remote locations. One system used to eliminate the need for laying cable is utilizing a radio-telemetry system which takes advantage of individual, remotely located terminal units which send the necessary data via a radio link to a central or master terminal unit, which then causes the appropriate corrective action to be initiated. The use of remote terminal units in communication with a central or master terminal unit is described in the Reese SPE paper, supra, and has been utilized by Amoco Pipeline Company, called the AMOCAMS system. With the AMOCAMS system, remote terminal units (RTU's) are located at pumping stations on a pipeline. The RTU's send data on flow rate, pressure, valve positions and the like via telephone lines or radio link to a master terminal unit (MTU) which calculates what, if any, corrective action is necessary and sends a control signal back to the RTU's to initiate the appropriate action. The use of RTU's and MTU's, such as an AMOCAM system, have not, prior to the present invention, been suggested or disclosed to be used to control the combining of fluids prior to injection into a wellbore.